Cytogenetic Characterization and Nuclear DNA Content of Diploid and Tetraploid Forms of Stokes Aster

Stokesia laevis (J. Hill) Greene is a herbaceous perennial native to the southeastern United States. Most cultivars of Stokesia are diploid (2n = 2x = 14) except for 'Omega Skyrocket', a tetraploid (2n = 4x = 28) form selected from a natural population. A comparative study of the karyotypes and meiotic behavior of diploid cultivars, seed-derived accessions of 'Omega Skyrocket', synthetically derived autotetraploids, and triploid progeny from these taxa strongly suggest that 'Omega Skyrocket' is an autotetraploid form of Stokesia. Total karyotype length, 161 μm and 293 μm, and average chromosome length, 11.5 μm and 10.5 μm, of the diploid cultivars and tetraploid accessions of 'Omega Skyrocket', respectively, were determined. The karyotype of the diploid cultivars consisted of eight metacentric (m) and six submetacentric (sm) chromosomes with average arm ratio values ranging from 1.12 to 2.06. The karyotype of 'Omega Skyrocket' consisted of 23 m chromosomes and 5 sm chromosomes with average arm ratio values ranging from 1.22 to 2.02. Meiotic pairing in the diploids was normal. No meiotic irregularities such as laggards or bridges were observed and disjunction was balanced (7:7). Accessions of 'Omega Skyrocket' demonstrated a high frequency (60%) of quadrivalent formation; however, later stages of meiosis were regular with balanced disjunction (14:14) occurring in 95% of the cells. Meiotic configurations in synthetically derived autotetraploids and triploid hybrids from crosses of diploid cultivars x 'Omega Skyrocket' consisted of univalents, bivalents, trivalents, quadrivalents, and pentavalents. Abnormalities, including laggards, unequal and/or premature disjunction, chromosome bridges, and chromosome stickiness were observed. Average nuclear 2C DNA content was 20.3 pg for the diploid cultivars and 39.9 pg for the newly synthesized autotetraploids. Average nuclear 2C DNA content for 'Omega Skyrocket' was 37.3 pg, which was 8.2% less than twice the average 2C DNA content of the diploid accessions and 6.4% less than the newly synthesized autotetraploids, suggesting that genomic downsizing in 'Omega Skyrocket' has occurred. Similarity of the karyotypes of the diploids and 'Omega Skyrocket' and the slight reduction in nuclear DNA content suggest that 'Omega Skyrocket' has diverged little from its original diploid progenitor.